The present application relates generally to electrophotographic image forming devices, and in particular to a toner transfer apparatus to control charge buildup in a toner image as the toner image passes through one or more image transfer stations.
Electrophotographic image forming devices, such as laser printers, facsimile machines, copiers, all-in-one devices, etc, are well known in the art. Color electrophotographic image forming devices may form a plurality of latent electrostatic images, develop each color plane image with toner particles, and ultimately transfer the color plane images to a media sheet and then fuse them to the media sheet using heat and pressure. Color electrophotographic image forming devices may be divided into two types by considering how toner is transferred to the media sheet. In a direct to media (DTM) type image forming device, the developed toner image of each color plane is successively transferred directly to the media sheet. In an intermediate transfer mechanism (ITM) type image forming device, the developed toner image of each color plane is successively transferred to an intermediate transfer mechanism, such as a belt, and then the full-color image is transferred to a media sheet at a secondary transfer location.
One known problem that particularly affects ITM type image forming devices is charge buildup on the developed toner on the ITM as the toner passes successively through high-voltage image transfer stations. Toner which has passed through multiple image transfer stations may be at a different charge than toner which has not passed through any additional image transfer stations. When the toner image is transferred to the media sheet at the secondary transfer location, the toner that is less charged may transfer at a lower voltage than more highly charged toner. In order to transfer the entire toner image, a voltage high enough to affect the transfer of the most highly charged toner is used. High transfer voltages may create a phenomenon known as Paschen breakdown. In Paschen breakdown, toner particles reverse polarity and their placement becomes unpredictable. The toner particles may even backtransfer from the media sheet to the ITM. Backtransfer detrimentally impacts image quality.